Developers of elastic shoe soles are confronted with the problem of storing energy in the sole of the shoe and releasing the energy in a manner which improves walking and running economy while at the same time achieving adequate shoe stability and cushioning.
Many examples of running shoes exist; however, very few, if any, have been embraced by runners and foot specialists. Studies of the dynamics of the human foot reveal that the longitudinal arch of a human foot, with its numerous joints and ligaments, is capable of returning 70% of the energy put into it and that it is far more efficient than the soles of the best existing running shoes. See Discovery, October 1989 (pp. 77-83). If runners could run with bare feet, many would prefer to do so. However, impact forces are too great and the foot needs protection, especially on hard surfaces.
Proposed spring designs for shoe soles have not been entirely satisfactory. The spring structures are not optimally light and efficient while still meeting biomechanical stiffness and stability requirements. These problems are overcome by the present invention. The inventors hereof have designed an optimal heel spring and forefoot spring. In addition, the inventors hereof have discovered that for heel-toe running, energy should be stored in the sole of the shoe at heel-strike and then, in turn, this stored energy should be released at a critical time and in a particular manner to enhance toe-off propulsion. A two and three coupled spring system was designed which achieves this heel to toe transfer of energy.
While purporting to provide for maximum performance, the latest technology in elastic shoe soles fails to achieve the ultimate design which permits maximum storage and return of energy while still achieving shoe stability and cushioning in a variety of locomotion modes ranging from heel-toe walking and running, flat-footed running (long-distance) and toe running (sprinting).
U.S. Pat. No. 4,941,273 (Gross) discloses an athletic shoe having a sole arrangement which contains an elastic band extending through a longitudinal passageway in the mid-sole. The purpose of this device is to create an artificial tendon which stores and releases energy during the running cycle. However, this heel spring design is inadequate for several reasons. Most importantly, the spring stiffness does not increase with increasing running speed, an essential heel spring performance requirement.
On the other hand U.S. Pat. No. 4,492,046 (Kosova) describes a sole of a running shoe having a wire spring. The wire spring serves to bias the anterior portion of the sole from the heel (back of shoe) forward to the arch region, separating the anterior of the sole into upper and lower portions. The objective of this device is to enhance the runner's performance by reducing impact at heel-strike and launching the runner forward into a comfortable stride. Unfortunately, this wire spring configuration could not be optimally light while at the same time adequately stiff for both running and walking. Additional material would have to be used within the wire frame to achieve adequate heel stiffnesses. This additional material would most likely reduce the overall heel spring efficiency.
The latest commercially available elastic shoes fail to completely address the desired attributes of a properly designed shoe with springs. For example, air bladders have been used in shoe soles in an attempt to increase walking and running economy. Researchers have found that it is difficult to achieve shoe stability while simultaneously achieving measurable increases in economy using air springs. Other commercially available shoes, although claiming to facilitate propulsion at toe-off, are incapable of storing substantial amounts of energy at heel-strike, allowing kinetic and potential energies to be lost to heat.
Similar challenges have confronted developers of lower-extremity prosthetic limbs. While it has effected substantial improvements, prosthetic research has so far focused almost exclusively on simulation or duplication of a natural foot in an attempt to provide the amputee with a normal gait and a greater degree of comfort. See U.S. Pat. No. 4,652,266 (Truesdell). A recent improvement emerging from research is the College Park Foot design disclosed in U.S. Pat. No. 4,892,554 (Robbinson). This design describes a prosthetic foot having an ankle member, a heel member and an elongated metatarsal-toe member coupled to each other for relative pivotal movements. The toe member is partially bifurcated at its forward end to provide independently flexible toe sections at the inner and outer sides of the foot. This design thus represents a three-point balance system achieving a stable support matching that of a natural foot.
One notable exception is the device disclosed in U.S. Pat. No. 4,822,363 (Phillips). This patent describes a composite prosthetic foot having a leg portion, a foot portion and a heel portion all rigidly joined and all three provided with substantial elasticity to allow return of energy absorbed and permit the amputee to engage in sports such as running and playing tennis. Understandably, this design has met with general approval from amputees who are sport enthusiasts, and at the same time enjoyed commercial success.
The above disclosed invention does not contain a mechanism whereby energy absorbed at heel-strike can be stored and later released at toe-off as does the two and three coupled spring systems disclosed herein.
The present invention represents a significant improvement over the prior art. The purpose of the present invention is to minimize energy losses during the impact phase and to use these energies to improve the runner's overall economy. The design can be incorporated into either a shoe for use with a natural foot or a foot prosthesis specially fabricated for amputees wishing to participate in athletic activities. Moreover, the present invention maximizes the runner's performance in all running modes including heel-toe running, sprinting and running flat-footed.